1. Technical Field
This invention relates generally to the field of ocean bottom environmental monitoring and particularly to equipments useful for determination of the amount of time that an underwater life catching trawl is actually dragged along the bottom.
Agencies responsible for regulation of ocean pollution require and rely on benthic (ocean bottom) trawls for fish and invertebrate samples in order to determine changes in population of local bottom species which might result from growing toxic ocean contamination. Many private and governmental policy decisions relating to ocean dumping, industrial pollution, and sewage disposal are based in part on the results of these bottom trawls. Inaccuracies of the basic data can at best lead to expensive retesting and at worst to flawed policy decisions. Are major problem associated with current benthic trawling is that of obtaining a reliable estimate of the amount of time that the net is actually in contact with the bottom, as opposed to "flying" above the bottom population. For example, if 50 purple sea urchins are collected in a particular location as compared with 100 during a previous quarterly sampling, it is necessary to determine whether the boards were flying half of the time or whether a possible toxic disaster is in the making.
2. Background Art
Typical sample tests require that a trawl net, having a 7.62 meter frontal opening mouth, be pulled along the ocean bottom at a speed between 2 and 2.5 knots for ten minutes. The distance supposedly covered along the bottom is about 770 m. The net mouth opening is vertically maintained by means of a metal chain weighted footrope and a headrope with floats sewn into the upper edge. Each side of the net is attached to a door, known as an "Otter Trawl Door", which is dragged upright on a runnered edge along the bottom. The function of the doors is to pull the trawl net down and to maintain the mouth of the net open to its regular swath. The doors are also attached to the tow cable which is connected to the trawling vessel by means of a four part chain bridle. The total catch from a trawl in each designated area is brought aboard the vessel, segregated by species and counted.
Benthic sampling is generally done with moderately sized vessels of 13 to 45 meters overall length, equipped with a deck winch and cable drum. Handling and shooting (putting overboard) of the trawl net assembly is done manually. When the trawl net is in the water and tracking smoothly, the towing cable is gradually let out to the desired length. The typical scope (length) of wire cable let out will be three to five times the water depth in the trawl area, which may run from 20 to 1000 meters. Most trawls in the Southern California area are made at depths between 20 to 150 meters.
Most trawl methods include making estimates of the bottom trawl time based upon measured total trawl time, length of tow wire payed out, and floor depth knowledge or measurement. Trawl duration is measured from the time that the winch stops deploying the trawl to the time that the winch starts retrieval. Distance travelled by the boat is measured with LORAN C or a comparable instrument, but it is not a reliable measure of on-bottom time.
The behavior of a particular trawl at a given station can vary substantially from the nominal expected performance because of wind, swell and wave effects on the towing vessel, descent and ascent rate variations of a particular board and net rigging, and current velocity changes down the entire water column.
The type of trawl board described above is illustrated in U.S. Pat. No. 3,299,560 to Luketa for Sectional Trawl Doors. There, each door comprises a door body having separate top and bottom edge sections, the latter of which ride upon the bottom, and an intermediate section. The sectional design is symmetric and either edge may be used downward on either the left or right side.
A variety of other patents have been issued which pertain to either improved trawling devices or to other equipments for environmental benthic monitoring. For example, in U.S. Pat No. 4,771,565, Shepard discloses a Bottom Trawl Roller which can be attached to the net in order to reduce the friction generated as a trawl net is dragged along the bottom during shrimp fishing. Another roller type is disclosed by Holden in U.S. Pat. No. 3,548,531 for a live bait dip net which may also be converted to trawling. Both improvements promote low flying.
U.S. Pat. No. 4,164,199 to Pequegnat discloses a Benthic Aquatic Biotal Monitor for monitoring the chronic impact of pollution, such as industrial and municipal wastes, of aquatic environments upon in situ samples of benthos. The conical enclosure includes a base portion including containers for retaining benthic sample and having permeable walls. A benthic dredge construction is shown in U.S. Pat. No. 3,762,078 in which a dredge is especially adapted for taking samples from the bottom of a body of water. A Submarine Device is described in U.S. Pat. No. 3,415,068 to Casey et al, wherein the apparatus includes a body adapted to be flooded to sink to the bottom of a body of water in order to agitate the bottom and collect benthic specimens.
All of these benthic collectors are relatively static and do not operate at towing speeds that would produce an off-bottom flying problem.